The US IOM, now National Academy of Sciences, looked at 9,000 papers and calls it ME/CFS, with the last part being "chronic fatigue syndrome."
I am seeing one of the top specialists in the world, and had fatigue as my top symptom, and he diagnosed me with ME/CFS. I see 2 neurologists, so there are definitely CNS features in my illness, too.
In the US, it's ME/CFS. I keep hearing Europeans dicker about it being ME vs. CFS, which seems counterproductive. It seems there are research papers coming out regularly from all over the world, Europe, Asia, Australia, and the US, using the label ME/CFS. And, PEM, post-exeerional malaise is a cardinal feature of ME/CFS, which involves fatigue.
Having had family members with Parkinson's, cancer, autism, and menral illness, what I've learned is that none of these diagnoses are pure and cast in stone. There are variations, different etiologies and different treatmrnts that work.
What we've learned about ME/CFS is that there's an infection variant, an autoimmune variant, a structural (CCI, etc.) variant, with metabolomic abnormalities, and vsriations and combinations of all of these. And, while we'd a like a magic pill that fixes us, personally, I don't believe that one pill is going to fix all of us with all these individual variations. Recently we learned that surgery can be a fix for some.
Everyone's disease is individual, with differences in individual genes, immune factors, environmental factors, etc. And it's likely there will need to be a robust toolbox of tools to apply to individuals to help them to a cure. It's vexing, but likely true...
As for cancer fatigue, I've attached a couple of papers discussing it. I question whether it's really so different - there are definite similarities. I haven't looked into MS fatigue, but I'd wager there are similarities there, too.